Recently, more audio equipment is being designed for higher power, resulting in a growing demand for speakers with higher maximum input power. The coil portion of the voice coil generates heat as a result of electrical resistance when the input signal is applied to the speaker and current flows in the coil portion. This heat is radiated through the voice coil bobbin, magnetic gap and other components. However, if excessive input power is applied to the speaker and a high current flows through the coil portion, insufficient radiation takes place and the coil portion will bum out due to the generation of excessive heat.
Therefore, it is necessary to improve the heat radiating function to permit increased dissipation of the heat generated in the coil portion of the voice coil, to improve the maximum input power of the speaker.
One known arrangement for a speaker with a better heat radiating function is a speaker unit disclosed in Japanese Utility Model Laid-open Patent No. S61-104698. More specifically, a through-hole is provided along the shaft direction of the center pole, and a heat sink made of non-magnetic material is disposed inside the hole. This speaker unit is installed in a cabinet and used as a speaker system.
The heat transfer path of the above speaker unit is explained with reference to FIG. 16, which shows a section view of a speaker system employing the speaker unit disclosed in Japanese Utility Model Laid-open Patent No. S61-104698. When the input signal is applied to the speaker, heat is generated as a result of the electrical resistance of a coil portion 11 of the voice coil. This heat conducts through a magnetic gap 15 to a top plate 13 and the yoke 14 thereof. The heat then reaches a heat sink 12 disposed inside a yoke 14, and is released to the inside of cabinet 16. Heat conductivity, which indicates how fast heat can travel, is approximately 0.02W/m.degree. K at the magnetic gap 15, 80 W/m.degree. K at the top plate 13 and the yoke 14, and 240 W/m.degree. K at the heat sink 12 (when it is made of aluminum).
Since the magnetic gap 15, which has low heat conductivity, is included in the heat transfer path, sufficient radiating effect cannot be expected due to the heat transfer loss.
In addition, heat released from the heat sink 12 will increase the temperature inside the cabinet 16. This results in a lower radiating effect due to the smaller temperature difference between the heat sink 12 and the inside of cabinet 16.
Furthermore, a die is needed for making the heat sink 12, resulting in higher cost.
Furthermore, since air passes through the narrow space created by the heat sink 12, an abnormal noise is generated by the moving air.